PhotoBeacon Localization System

The PhotoBeacon system is meant to provide a simple means for
localizing many (>50) small, autonomous mobile robots (in
particular, we are targeting the CotsBots).
This system works
using triangulation, or measuring the angles between robots. However,
in
addition to determining the bearing of other robots for localization
purposes,
the PhotoBeacon system can also be used to map obstacles or free space
and provides a 1 kbps optical communication link between robots. In
order to reduce the computational complexity generally required for
localization, the PhotoBeacon system moves much of the complexity to
hardware. The system is composed of four basic components: a high-power
LED beacon to transmit, a fisheye lens to capture transmitted light
from other robots in the horizontal plane, a custom CMOS sensor chip to
detect signals from other robots, and a printed circuit board (PCB)
with microcontroller to control the other components.

Design concept
for PhotoBeacon
system consisting of four separate components: the high power IR LEDs,
a fisheye lens, the custom PhotoBeacon IC sensor, and a PCB with 8-bit
microcontroller.

Die photo of PhotoBeacon IC fabricated in 0.25 um
CMOS
process.

PhotoBeacon IC

The custom PhotoBeacon Sensor IC is perhaps the most
interesting and challenging design aspect of this system. This sensor
needs to be able to detect very low power incident signals and resolve
the relative angle to approximately 1º. In addition, the chip
itself should be low power and provide a relatively simple interface to
any external controllers.

The PhotoBeacon
IC uses 256 photodiodes arrayed in the shape of an annulus to detect
incoming signals. If light is focused onto a single pixel, the angle of
incidence may be determined to approximately 1.4º. These
photodiodes are multiplexed into a modified version of the 1 Mbps
optical receiver designed by Brian Leibowitz which includes
three switchable sense resistors and a gain stage which can provide up
to 33 dB of gain. In addition, it also provides a way to discard DC
current from ambient light. Finally, the receiver
and multiplexor are controllable over a simple 3-wire serial interface.
Simulations show the receiver detecting 1 kbps signals at under 100pW
received power (equivalent to high power LEDs at 10m range).